(1) Field of the Invention
This invention relates to a 3D image reconstructing method for a positron CT apparatus (hereinafter referred to as “PET apparatus” as appropriate) and to a positron CT apparatus for acquiring sinograms for each combination of ring pairs by detecting coincidence counting data of the ring pairs in each moving position, while moving a multi-ring detector having ring type detectors arranged in an axial direction and a bed having an object under examination placed thereon, relative to each other axially of the multi-ring detector, and performing 3D image reconstruction for RI distribution image reconstruction based on these sinograms. Especially, the invention relates to a technique for improving the quality of RI distribution images.
(2) Description of the Related Art
Conventionally, a PET apparatus that performs an image reconstruction process for RI distribution images based on sinograms acquired through 3D data acquisition moves a multi-ring detector having ring type detectors arranged in an axial direction and a bed having an object under examination placed thereon, axially of the multi-ring detector, for example, relative to each other intermittently at intervals of the ring type detectors. The apparatus detects coincidence counting data of ring pairs in each moving position to collect sinograms for each combination of ring pairs, and performs an image reconstruction process for RI distribution images based on these sinograms.
The conventional PET apparatus usually performs 3D iterative reconstruction for imaging after acquiring all sinograms through 3D data acquisition. However, the sinograms acquired through 3D data acquisition constitute an immense amount of data. Besides, 3D image reconstruction accompanied by 3D projection and back projection processes is started after completion of the sinogram acquisition. The amount of data stored is inevitably huge, and a long time is taken from the start of an examination to the end of imaging.
A different conventional PET apparatus acquires 3D data while adding sinograms to sinograms obtained in an immediately preceding position as sinograms of each ring pair acquired from the same site of the object, and reads successively sinograms having been added, in parallel with the 3D data acquisition process. After the 3D data is converted into 2D data, 2D iterative reconstruction is performed in parallel with the 3D data acquisition process (see Japanese Unexamined Patent Publication No. 2004-61113 (pages 4-8, FIGS. 1-4)).
This prior apparatus, which has been proposed by Inventor herein, performs addition and reading of sinograms and an image reconstruction process in parallel with the 3D data acquisition process. Thus, the apparatus can reduce the amount of data stored and shorten the time taken from the start of an examination to the end of imaging.
However, the above PET apparatus tends to cause image quality deterioration due to a drop in resolution and lowering of S/N ratio. Since this conventional positron apparatus converts 3D data into 2D data, it is difficult to avoid a drop in resolution due to errors occurring with the conversion from 3D data to 2D data. Further, since images are reconstructed from the 2D data, the apparatus cannot directly use 3D data physical models such as detector response, attenuation correction and statistical property in time of image reconstruction. The apparatus has limitations for improvement in the S/N ratio of reconstructed images.